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May 19, 2008 -- More and more kidney cancers in the United States are being diagnosed at early, highly treatable stages, and more patients are surviving the disease, a national study confirms.

In just over a decade, between 1993 and 2004, the percentage of kidney cancers diagnosed at the earliest "stage I" level rose from 43% to 57%.

The average size of a stage I tumor decreased from 4.1 cm to 3.6 cm during roughly the same period. There was a 3.3% increase in survival among patients of all stages from 1993 to 1998.

Calling the findings "quite dramatic," the study's lead researcher tells WebMD that they were also something of a surprise.

About 50,000 new cases of kidney cancers are discovered each year in the United States.

"A 3% increase in survival may seem like a small number, but it represents a lot of patients," says Christopher J. Kane, MD, who is chief of urology at the University of California, San Diego.

More Kidney Cancers, Fewer Deaths

The overall incidence of kidney cancer is on the rise in the U.S., but much of the increase has been seen in early-stage disease.

Kane says there had been signs that kidney cancers were being diagnosed earlier, but the magnitude of the change has not been known.

In the largest study ever to address the question, the researchers reviewed the medical records of more than 200,000 kidney cancer patients registered in a national cancer database.

The findings confirm that a smaller percentage of cancers are being discovered after the disease has spread beyond the kidneys.

A 14% rise in the diagnoses of the earliest, stage I cancers between 1993 and 2004 was accompanied by an 8% decline in the deadliest stage IV cancers.

The five-year survival for stage IV disease is about 10%, compared to close to 90% for stage I kidney cancer.

Roughly 19% of the kidney cancers diagnosed in the U.S. in 2004 involved stage IV disease, compared to about 27% in 1993.

During this time, the proportion of stage II and stage III cancers also declined, from 12% to 10% and 18% to 14%, respectively.

"This evidence suggests that all cases are migrating from later stages to earlier stages over the period of observation," Kane and colleagues write in the online version of the journal Cancer.

The Impact of Diagnostic Imaging

Though the study did not examine the reasons for the shift, experts say the most likely explanation is the increasing use of diagnostic imaging such as ultrasound, CT scans, and MRI for the detection of other health problems.

"People are being screened for other reasons, but imaging often picks up incidental masses on the kidneys when they are still quite small and well before there are symptoms," Kane says.

Imaging is not routinely used to screen for kidney cancer, and its value as a screening tool has not been well studied.

"Abdominal imaging is pretty expensive and kidney cancer is pretty uncommon," Kane says. "So we would probably end up doing a lot of imaging without picking up a lot of kidney cancers."

American Cancer Society Director of Surveillance Research Elizabeth Ward, PhD, agrees that the value of diagnostic imaging as a screening tool for kidney cancer remains to be determined.

"When we recommend cancer screening for asymptomatic people it is because we have the clinical trials to show that screening improves survival," she says. "Those types of studies have not been done."